Since a time division duplex CDMA/TDMA mobile communication system may flexibly control and change the proportion between the length of uplink data transmission period and the length of downlink data transmission period, it is widely adopted for data transmission on asymmetrical services. Wherein, uplink/downlink services are carried in different time slots at the same carrier. The carrying capacity of uplink/downlink services may be flexibly adjusted by assigning amount of uplink/downlink time slots so as to match the proportion of uplink/downlink services. Thereby, the system capacity loss caused by asymmetry in services will be decreased to the minimum; the system capacity will be increased, and the optimal spectrum efficiency will be achieved.
However, in the case of co-frequency operation, if the division of time slots among adjacent cells is not the same, namely, adjacent cells adopt different uplink/downlink switching points, some time slots of adjacent cells may carry services in different directions, such time slots are called cross slots. So called cross-slot interference means that downlink or uplink signals in one cell may interfere to uplink or downlink signals in co-frequency adjacent cells. As shown in FIG. 1, time slots enclosed in the dashed block are used for uplink services in cell 1 and used for downlink services in cell 2. When a base station in cell 1 receives an uplink signal of cell 1, the base station may receive strong interference caused by downlink signals from a base station in cell 2, this kind of interference is called base station-to-base station cross-slot interference. For a terminal located at boundary between two cells, for example terminal A in cell 2, terminal A may receive interference caused by the uplink signal from terminal B in cell 1, this kind of interference is called terminal-to-terminal cross-slot interference. Due to large transmission power of base stations, high antenna gain and small path loss, the base station-to-base station interference is more significant. The base station-to-base station interference may cause that cells carrying uplink services in cross slots hardly work, which will seriously deteriorate the communication quality and system performance.
Conventionally, methods are provided to avoid cell-to-cell cross-slot interference. One method is that adjacent cells adopt the same frame synchronization and the same uplink/downlink time slot switching point under the control of a radio network controller (RNC). FIG. 2 is a flowchart showing a configuration method for adopting the same time slot switching point to avoid the cross-slot interference in prior art. However, since the asymmetry characteristics of uplink/downlink services is not exactly the same among different cells, the configuration method of the same switching point can not efficiently utilize spectrum resource by using this method. Another method is a configuration method of adopting different time slot switching points, when there are cross slots, RAN usually sacrifices capacity during allocating resources, namely, avoiding the use of cross slots. FIG. 3 is a flowchart showing a configuration method of sacrificing capacity adopted to avoid the cross-slot interference in prior art. However, both of the methods may not completely utilize the system resource.
Also, in prior art, according to a cell setup process shown in FIG. 4, RNC notifies certain base station (Node B) controlled by the cell of its own configuration information, such as cell ID, absolute frequency ID, maximum transmission frequency, synchronization channel configuration, configuration information of common channel carrying broadcasting information, time slot configuration information and the like, wherein the time slot configuration information related to the present invention include the number of time slots, time slot direction (uplink or downlink), and time slot state (activated or not). According to the cell setup process described above, the base station may determine a frame structure for the local cell, namely, the position of the time slot switching point. However, in existing systems, a base station may only obtain its own configuration information and may not acquire those of adjacent cells, so that the cross-slot interference may not be suppressed or eliminated by using a corresponding method. In existing time-slotted CDMA systems, joint detection technique is commonly used to suppress interference (mainly intersymbol interference and multi-access interference) to improve the performance of CDMA systems. The joint detection method is implemented by using information of all users' transmitting signals in the local cell and information of channel responses of these signals to regard the signal detection as a united and related joint detection process.
Existing receiver algorithms are mostly single cell joint detection algorithms. The algorithms only use the structure information (including spreading code and channel response) of users' transmitting signals in the local cell, and regards interference signals from other cells as white Gaussian noise over time. Therefore, single cell joint detection algorithms can efficiently suppress intersymbol interference and multi-access interference in the local cell.
A single cell joint detection process in a TD-SCDMA system is shown in details as following.
Firstly, a signal model received by a single cell receiver in a time-slot system is obtained:e=Ad+n  (1)
Where e represents sampling data for signals received by the receiver, d represents transmitting data, n represents received noise, and matrix A represents a transmission matrix. The transmission matrix A consists of combined channel impulse response b(k) of code channels, where k is code channel ID, it is supposed that there are L code channels in total (matrix A may be calculated by b(k), b(k) is one column in each block arranged on diagonal of the matrix, the detail calculation method refers to Chinese patent No. 02148622.0, entitled “Method of Applying Long Scrambling Code in Joint Detection System”).
Each vector b(k) is a combined channel response corresponding to a code channel with the code channel ID of k:b(k)=C(k)⊕h(k) k=1 . . . L  (2)
Where C(k) is a spreading code of the code channel with the code channel ID of k, ⊕ is a convolution operator, h(k) is channel response of the code channel with the code channel ID of k, which is obtained by using a midamble (middle code) to perform the channel estimation.
Then, joint detection may be performed by using the above mentioned information. There are various joint detection algorithms that may be interference cancellation method, block linear equalization method, or a combination thereof. For the block linear equalization method, the soft symbol estimated by transmitting data d after demodulation is:{circumflex over (d)}=(T)−1A*TRn−1e  (3)
Where matrix T is calculated with Formula (4):
                    T        =                  {                                                    I                                            MF                                                                                                          A                                          *                      T                                                        ⁢                                      R                    n                                          -                      1                                                        ⁢                  A                                                                                                  Z                    ⁢                                                                                  ⁢                    F                                    -                                      B                    ⁢                                                                                  ⁢                    L                    ⁢                                                                                  ⁢                    E                                                                                                                                                                  A                                              *                        T                                                              ⁢                                          R                      n                                              -                        1                                                              ⁢                    A                                    +                                      R                    d                                          -                      1                                                                                                                                        M                    ⁢                                                                                  ⁢                    M                    ⁢                                                                                  ⁢                    S                    ⁢                                                                                  ⁢                    E                                    -                                      B                    ⁢                                                                                  ⁢                    L                    ⁢                                                                                  ⁢                    E                                                                                                          (        4        )            
Where A*T is a conjugate transpose matrix of the transmission matrix A, and Rd=E{d·d*T} is a covariance matrix of data sequence d, d*T is a conjugate transpose sequence of matrix d, in respect of data sequences which are independent to each other, Rd=I; Rn=E{n·n*T} is a covariance matrix of noise sequence n, and n*T is a conjugate transpose matrix of matrix n. In respect of noise sequences which are stable and independent to each other (such as white noise), Rn=σ2I, where I represents a unit matrix.
Under the condition of Rd=I and Rn=σ2I, formulas (3) and (4) may be simply represented as:
                              d          ^                =                                            (              T              )                                      -              1                                ⁢                      A                          *              T                                ⁢          e                                    (        5        )                                T        =                  {                                                    I                                            MF                                                                                                          A                                          *                      T                                                        ⁢                  A                                                                                                  Z                    ⁢                                                                                  ⁢                    F                                    -                                      B                    ⁢                                                                                  ⁢                    L                    ⁢                                                                                  ⁢                    E                                                                                                                                                                  A                                              *                        T                                                              ⁢                    A                                    +                                                            σ                      2                                        ⁢                    I                                                                                                                    M                    ⁢                                                                                  ⁢                    M                    ⁢                                                                                  ⁢                    S                    ⁢                                                                                  ⁢                    E                                    -                                      B                    ⁢                                                                                  ⁢                    L                    ⁢                                                                                  ⁢                    E                                                                                                          (        6        )            
MF in Formulas (4) and (6) is matched filtering, corresponding to matched filtering method; ZF-BLE is a Zero-Forcing Block Linear Equalization method, corresponding to a linear solution of the maximum likelihood; and MMSE-BLE is a Minimum Mean Square Error Block Linear Equalization method, corresponding to a linear solution of the minimum mean square error. One of the three methods described above may be chosen to solve T, and the second one ZF-BLE or the third one MMSE-BLE is usually chosen.
However, in the case of co-frequency networking, there is strong interference among signals of co-frequency adjacent cells that will significantly affect the system performance. Particularly, at the boundary between co-frequency adjacent cells, the co-frequency interference is usually the most important interference. Meanwhile, the single cell joint detection method may do nothing about the co-frequency interference among adjacent cells, if there is the co-frequency interference among adjacent cells, the system performance will be significantly deteriorated.
Focusing on the interference among co-frequency adjacent cells, in Chinese patent application No. 200410080196.6, entitled “Method for Multi-cell joint detection in time-slotted CDMA System” and filed by the same assignee of the present application, a method is proposed to use a channel estimation method for multiple code-sets (disclosed in Chinese patent application No. 03100670.1, entitled “Channel Estimation Method for Multiple Code-Sets in time-slotted CDMA System” and file by the same assignee of the present invention) to perform channel estimation on each adjacent cell, to adopt appropriate code channel grouping, and to implement joint detection according to the channel estimation result and the code channel grouping result. The multi-cell joint detection method fully utilizes structure information of signals from multiple cells to efficiently suppress multi-access interference among co-frequency adjacent cells, and the system performance of co-frequency adjacent cells is improved. However, since the cross-slot interference is not considered, in the case shown in FIG. 1, the method may not fully utilize the information provided by cross slots and may not achieve a good application effect.